So Orange are bringing out a new Crush which is advertised as a "mullet bike", 29er on the front and 27.5 on the back. I'm guessing you can't run it with both 29er wheels due to the clearance at the rear, seems a strange decision as not many people will have a set of 29er and 27.5 wheels to build the frame only option.. £550 an Aluminium frame which can only take 27.5 and 29er wheels seems to really limit your potential market ?
https://www.orangebikes.co.uk/bikes/crush-mx-frame/2021
There's a crush 29 if you want a 29er
there's nothing to stop you building the frame only up as a 27.5" bike - nothing about the frame dictates the front wheel size other than how they've measured the geometry.
I don't think mulleting a hardtail makes any sense - on a DH bike it might make sense if you can't get the travel you need with a big wheel in a smaller frame, but with a hardtail you're just going to have one wheel which is suspended also rolling better over everything, and a non-suspended wheel getting hooked up on everything more. Zero value in that..